1. Field of the Invention
The present invention relates to a developer apparatus which develops an electric latent image on the surface of a photoreceptor by using a developer containing a magnetic carrier and a non-magnetic toner.
2. Description of the Prior Art
Typically, the toner concentration of a developer apparatus of this type is determined based on a result of a comparison between a predetermined reference value and a toner-to-carrier ratio in a mixture of toner and carrier detected by a permeability sensor. The output of the permeability sensor increases as the toner concentration decreases, and decreases as the toner concentration increases. Therefore, when the sensor output is higher than the reference value, toner is replenished to maintain the toner concentration constant.
It is known that when the power is turned on, the developer in the developer unit is of a small volume and is therefore of a high concentration so that the output of the sensor is high for the toner concentration. Therefore, in such a case, if the output of the sensor is used for the comparison with the reference value, toner is excessively replenished to increase the toner concentration.
To solve this problem, when the power is turned on, the reference value is set to a high value at first and then stepwisely decreased therefrom. By this method, the malfunction of the toner concentration control is tentatively avoided when the power is turned on.
In the case of an electrographic copying machine, however, after the power is turned on, the copying machine is frequently left in a copy ready condition for a long time whether copying is necessary or not. For this reason, there exists a considerable period of time during which copying is not performed (therefore, the developer unit does not operate). In such an unoperated period, the agitation is not performed in the developer unit.
For this reason, although not as apparent as when the power is turned on, the air in the developer escapes therefrom during the unoperated period to decrease the volume of the developer, so that the sensor detects the toner concentration to be lower than the actual concentration when copying is performed. If toner is excessively replenished accordingly, the toner concentration is abnormally high, so that toner may scatter or fogging may occur on images.